1. Field of the Invention
The present invention relates to an X-ray computed tomographic apparatus which scans a patient by helical scan, and which reconstructs image data by ECG gated reconstruction on the basis of obtained data.
2. Description of the Related Art
An X-ray computed tomographic apparatus offers information about a patient, in terms of an image on the basis of the intensities of X-rays transmitted through a patient. It plays important roles in a large number of medical actions including the diagnoses and medical treatments of diseases, operation plans, etc.
Especially in the examination of the heart of fast motion as uses the X-ray computed tomographic apparatus, it is one of important problems to enhance the temporal resolution of an image. A major countermeasure against the problem is to shorten a time period per revolution of an X-ray tube, that is, to heighten the rotational speed of the X-ray tube. The rotational speed is also called “gantry speed” or “scan speed”.
A technique which can realize a temporal resolution higher than a rotational speed, is half reconstruction. As is well known, the half reconstruction reconstructs image data by the acquisition of projection data and the supplementation of the opposite data, the projection data being acquired while, as shown in FIG. 9, an X-ray tube rotates in a range of 180°+α (where α denotes a fan angle) around the specified phase of the motion of the heart as designated by an operator, usually a position which is expressed in percent with 100 being the cardiac period of an electrocardiogram from a P wave to the next P wave. With the half reconstruction, a time period which is required for the X-ray tube to rotate in the range of 180°+α is recognized as the temporal resolution of the image data.
A technique which enhances the temporal resolution of an image more than the half reconstruction is segment reconstruction (also termed “ECG gated reconstruction”). The segment reconstruction the number of segments of which is 3 as shown in FIG. 10, features that projection data in ranges of 60°+α/3 are collected from within three continuous or discrete heart beats, thereby to make up projection data in the range of 180°+α. The ideal temporal resolution of the segment reconstruction is given as a time period which is required for an X-ray tube to rotate in the range of 60°+α/3.
The ideal temporal resolution is a temporal resolution under a specified situation in which the rotational period of the X-ray tube ideally shifts relatively to a cardiac period. In most cases where the shifts of the rotational period of the X-ray tube relative to the cardiac period are not ideal, temporal resolutions lower in accordance with the shifts of the rotational period of the X-ray tube relative to the cardiac period. The worst case takes place when the rotational period of the X-ray tube and the cardiac period have synchronized completely. In that case, the number of segments automatically becomes one, that is, the segment reconstruction completely veers to the half reconstruction, and the temporal resolution agrees with that in the half reconstruction.
In this manner, in the segment reconstruction, the temporal resolution fluctuates in accordance with the shift of the rotational period of the X-ray tube relative to the cardiac period, and hence, it is difficult to be intuitively grasped. In a case where helical scan is employed conjointly with the segment reconstruction, restrictions concerning an effective slice thickness are imposed, so that the temporal resolution is difficult to be grasped.